Hi Carolyn. Basically, when selling minerals you are selling your ownership in all of the oil, gas, and/or other minerals in the ground under your tract of ownership. Mineral rights are most often perpetual property rights (at least in Texas) but can be for some limited period of time if they were conveyed that way in the past. On the other hand, an oil and gas lease is only a contract between a mineral owner and oil company allowing the oil company to extract said mineral owner’s property out of the ground in exchange for a bonus paid upfront at signing and royalty on production if it occurs. Leases are most often limited in duration but nearly always remain effective as long as production continues on the tract. They are not renegotiable unless production ceases for a period of time adequate to terminate the old lease and make your minerals open for a new lease again. If you own unleased minerals in Martin County they are likely still very valuable as there is a lot of oil and gas under those sections. Many buyers still want to buy those assets in the ground even though there isn’t any income at this time.
People talking about different net acreage numbers in your sections is likely due to quick calculations they’ve done from appraisal district data and/or what deed records they’ve found so far. If you were to sell some or all of the mineral rights you would definitely want to get a professional report of your ownership before closing.
Everything above is general for simplicity and brevity, but I hope provides a good top-level overview. I invite others to add anything I’ve omitted and make any corrections.